Volcanoes
Mt. St. Helens after eruption
What Causes volcanoes?
Upward force of magma
because it is less dense than
rock
Mt. St Helens before eruption
Volcanoes terms • Vent is where magma moves onto earth’s
surface
• Crater is a steep walled depression around a
vent
• Plate Boundaries
– Divergent plate boundaries –
where plates are moving apart
– Convergent boundaries – where
plates are moving together
– Hot spots – hot areas in the
mantle
• Pacific ring of fire is on plate
boundaries
Causes of Volcanoes
Earthquakes Worldwide
Mid-Plate Hotspots
Mid-Plate Hotspots
Types of eruptions
• Explosive and
violent
• Quiet lava flows
• Combination of the
two
Volcanoes
Volcanoes Time lapse of the eruption
• Causes of variation
–Trapped gases
–Composition of magma
Quiet lava flow
Volcanoes
Quiet
lava
flows
Types of Magma • Basaltic magma is very fluid
and produces quiet eruptions
• Granitic magma – causes
violent eruptions
– Gets trapped in vents
– Doesn’t flow
– Gases build up
– Magma water content causes
different amounts of energy
release
water content also influences
the violence of the explosion
Bridger mountains
Forms of volcanoes
Shield Volcano
Cinder cone volcano
Composite volcano
Shield ( Basic) Cone
Shield Volcanoes are enormous features
built up only from layers of lava. They
produce lots of lava but they tent not to
erupt violently.
Mauna Loa Hawaii -USA
250 miles
10,000m
Composite volcano to scale
Layers of Lava Lava Flow
• quiet eruptions that cause gentle sloping sides
Shield volcano
Volcanoes
Shield
Ash and Cinder Cone
Paracutin - Mexico
Layers of Ash
and Cinder
Volcanic Bombs
Pipe
The volcano is built
up of layers of ASH.
When it erupts it is
normally with great
explosive force.
3,000m
• explosive eruptions that throw lava high in
the air, the lava cools and forms different
sizes of volcanic material called tephra
which falls into a cinder cone
Cinder cone volcano
• vary between quiet and violent going from
tephra layer to quiet lava flow
Composite volcano
Volcanoes
Composite Cone
The volcano is built up
of alternate layers of
LAVA and ASH. They can
explode with great
violence.
Mt St Helens - USA
Conelet
Dyke
Crater
Ash layer
Lava Layer Flank
eruption
Lava
Flow
Crust
Pyroclastic Flow
Igneous Rock Features
• Batholiths – magma cools before reaching surface
• Dikes & Sills
– Dike – magma squeezed into vertical cracks and cools
– Sill – magma squeezes into a horizontal crack
Other features
Volcanic neck • magma cools in a
volcanic vent
Devils Tower, Wyoming
• collapsed volcanoes
• caldera is a large, depression at the top
of a volcano
– formed when magma is withdrawn from
magma reservoir
– The removal of magma may result in loss of
structural support for the overlying rock,
leading to collapse and formation of a
depression.
Caldera
Quiz
1. What are the three types of lava flows?
2. What are the three types of volcanoes
and what type of lava flow cause each?
3. What are the three causes of volcanoes?
4. Name three geological formations that
are the result of volcanoes.
5. What type of lava causes quiet lava
flows?
• Evidence for Continental drift (The idea that the
continents have moved to the position they are
in now)
• Wegener proposed the Idea in 1912 at the time
there was opposition
• Pangaea – super continent
Plate Tectonics – Continents are moving
Alfred Wegener
_________
proposed the
theory that the
crustal plates are
moving over the
mantle. This was
supported by
fossil and rock
type evidence;
also matching of
coastline shapes.
Wegener
Continental Drift
Pangea
The
break up
of
Pangea
Evidence to Support the continental
drift theory • Puzzle like fitting
together
• Fossils that are alike are found on different continents that were thought to be once connected
• Similar climates in fossil record
• Similar rock on different continents that were once connected.
Similar Fossils
Our first evidence of tectonic motion is based on similar fossils and
rock types on opposing sides of the ocean
Puzzle pieces
Tectonic Plates
Today plate boundaries are determined by examining
the location of volcanoes and earthquakes.
Volcanoes result from the friction (heat) of the plates
motion.
Earthquakes occur where plate rub against one another
Plate Boundaries
• Cracks in the plates
are called faults
Theory of plate tectonics
• the earth’s crust and upper mantle is broken in
to sections that move
• Plate Boundaries
– Divergent – moving apart
– Convergent – moving together
– Transform fault boundary
Plate Boundaries
• Divergent Boundary – moving _____
• Convergent Boundary – moving ________
• Transform Fault Boundary – moving
________________________
apart
together
sideways past each other
How could continents move?
• Spreading seafloor – Youngest rocks are in the middle
– Reverse polarity of magnetic fields of rocks laid down p. 301
Sea floor spreading
• Large continents begin to crack and split apart
• The gaps fill with water
• Small seas become oceans
• The mid ocean ridge continues to produce new crust
Tectonic Plates
Earthquakes Worldwide
Why do the Plates Move?
• No single idea explains everything but we can identify several forces that contribute to the movement of the plates. – Slab pull
• The sinking of the cooled dense oceanic plates pulls on the rest of the plate
– Ridge rises • The material deposited on the top of the ridge slides downs
from the rise pushing on the plate
– Convection • Movement within the mantle could be part of the driving force
behind the motion of the plates.
Causes of Platte Tectonics
• Convection
Currents
Results of Plate
movement
Convection Currents
The force responsible for plate movement is __________.
• How could the continents drift?
What Causes the Plates to Move?
• Convection- hot
magma rises, cool
magma sinks (like a
lava lamp)
Results of Plates moving
• Rift valleys are the result of divergent
plate boundaries
• Mountains arcs and volcanoes tend to be
the results of convergent boundaries
• Strike slip faults cause earth quakes
Types of Plate collisions
• Ocean plate – continental plate
collision
– Creates subduction zones
• Ocean plate – ocean plate collision
– Trenches to form
• Continental – continental collision
– Mountain ranges formed
Divergent boundary of two continental plates.
Creates a __________. Example: _____________ rift valley East African Rift
Convergent boundary of two oceanic plates.
Creates an ________ and a _____. Example: _____ island arc trench Japan
Convergent boundary of an oceanic plate and a
continental plate. Forms a _______ mountain range
and a ______. Examples: _______ or _______ Mts volcanic
trench Cascades Andes
Convergent boundary of two continental plates. Forms
a ______ mountain range. Examples: ___________,
Alps, ______________ folded Himalayas
Appalachians
Transform-fault boundary where the North American
and Pacific plates are moving ____ each other.
Example: ________________ in California
past
San Andreas Fault
Plate Boundaries Review
• Places where plates move apart are called
_____________ boundaries.
• When continental plates diverge a ___________ is
formed.
• When two oceanic plates converge what is created?
_________________
• The Appalachians formed mainly from continental
plate collisions and therefore are a __________
mountain range.
• The force moving the plates is ____________ .
Convection currents
divergent
rift valley
an island arc and a trench
folded
Andes Mountains,
South America